Add RemoteCallbacks.push_transfer_progress

[jorio]

The existing callback RemoteCallbacks.transfer_progress is only called during fetches, not pushes.
This PR exposes libgit2's push_transfer_progress callback, allowing users to:

• Report progress during a push;
• Abort a push by raising an exception in the callback.

Note: I had to rewrite test_push_options to use a real RemoteCallbacks object instead of a mock object (the mock wasn't complete enough to work with my additions to git_push_options)